On Aug 19, 2009, at 11:39 AM, Stephen A. Lawrence wrote:
Two very quick thoughts, probably both bogus:
1) You're striking an arc inside the bearing, and the controlling
thing
is the voltage; below the critical voltage needed to strike the (very
short!) arc the resistance is much higher
Yes. This is in fact what I expected to happen when I put the
graphite into the bearing. I hoped the arcing of that kind would be
less damaging to the bearing. It did in fact pretty much eliminate
the "growl" or "grind" sound that could be heard when power was
applied. This shows up in the videos I think. I look forward to
cutting the bearings open to see what they look like. I've held off
on that until I can get to Anchorage to buy some more. Thankfully
they are only about $7 a piece. When I get the next set I'll check
resistance after the washes, but without graphite.
Or....
2) Somewhere in the bearing, at the junction between two materials
(like
between the balls and the race, or maybe between the graphite lube and
something), you've got an accidental diode, with a junction drop of a
couple volts
Yes, it occurred to me as I was testing that the thing is acting like
a zener diode at low currents. One thing that occurred to me is a
surface pacification layer existing somewhere. That could be caused
by most anything. Possibly an aluminum junction somewhere. I'll have
to try to hunt down the surface where it is happening.
Well anyway those are the things which came to mind first.
Seems to make sense to me. It is odd the voltage drop is almost
constant up into a very high current range. It is only at large
currents and high rpms the back-emf shows up. It is also only at
large currents the motor runs. If it is acting as diode, it is a
pretty darn good and cheap diode, though no telling what the lifetime
of it is.
Best regards,
Horace Heffner
http://www.mtaonline.net/~hheffner/
